Since the introduction of highly active antiretroviral therapy (ART) in 1996, there has been a dramatic reduction in morbidity and mortality among those living with HIV. However, chronic liver disease due to coinfection with hepatitis B (HBV) or C (HCV) virus has emerged as the second leading cause of mortality among HIV-infected persons. The natural history of HBV infection is altered in those with HIV. Current guidelines recommend that most coinfected patients be treated for both HIV and HBV infection using combinations of ART that include tenofovir (TDF). Despite widespread adoption in the US, the effect of this regimen on long-term outcomes of HBV disease such as histologic severity, progression, and risk of emergence of resistant HBV variants, and the long term risks of TDF therapy remains unanswered. Further investigation is required to address the following important questions: (1) what is the proportion of HIV-coinfected patients who have incomplete viral suppression on TDF? (2) is incomplete suppression of HBV acceptable in HIV coinfected persons and if so, what threshold HBV DNA level constitutes an adequate clinical goal? (3) in view of the lack of acceptance of liver biopsy among HIV practitioners, can noninvasive markers accurately assess HBV disease activity and the impact of ART on disease progression? (4) What are the long term risks of TDF-based therapy for HBV in HIV coinfection? In short, what are the risks and benefits of TDF-based therapy for CHB in patients with HIV coinfection? The NIH Hepatitis B Research Network (HBRN) is the first major effort to elucidate the natural history and treatment outcomes of persons with chronic HBV the US. The HBRN will not address the critical issue of HBV liver disease progression in HIV-infected persons because patients with HIV coinfection will be excluded. The current proposal, an approved ancillary study of the HBRN, offers a unique opportunity to fill major gaps in HBV-HIV knowledge and to compare HBV-HIV infected persons to those with HBV monoinfection participating in the HBRN. No other funded research network in North America has the expertise, patient population, and structure to carry out the proposed studies. The Specific Aims are: 1. Define the problem. We will clinically, histologically, serologically, and virologically characterize a well-defined cohort of HBV-HIV patients in North America in a cross-sectional manner; 2. Define the benefit of long term therapy. We will longitudinally determine the impact of complete vs. incomplete viral suppression on clinical and serologic outcomes, and histologic progression by paired biopsy and 2a. Define a threshold HBV DNA level associated with disease progression; 2b. Establish the utility of noninvasive assessment of hepatic fibrosis compared with biopsy; and 2c. Define the frequency of genotypic and phenotypic TDF resistance with long term therapy; and finally 3. Define the risk of long term therapy. We will assess the long term renal and bone effects of TDF-based therapy in the HBV-HIV cohort. Collectively, this study will fulfill many of the key priorities outlined in the NIH Action Plan fo Liver Disease for HBV-HIV coinfection.